1. Field of the Invention
The present invention relates to a quartz crystal unit, and more particularly to a crystal unit having a quartz crystal blank hermetically sealed in a casing by joining a metal cover to a casing body with a metal brazing material by, for example, seam welding.
2. Description of the Related Art
Crystal units having at least a crystal blank hermetically sealed in a casing are used as a reference source of frequency or time in oscillators for various electronic devices. The casings of the crystal units are required to have a high sealing capability for keeping the oscillation frequency stable, and needs to have a reliable hermetical sealing structure.
FIG. 1A is a cross-sectional view of the structure of a conventional crystal unit, and FIG. 1B is an enlarged fragmentary cross-sectional view of a metal cover used in the conventional crystal unit.
The conventional crystal unit has crystal blank 2 housed in a cavity defined in casing body 1 and metal cover 3 placed over the opening of the cavity to hermetically seal crystal blank 2 in the cavity. Casing body 1 is made of laminated ceramics, and has external terminals (not shown) disposed on outer surfaces (bottom and side surfaces) thereof for mounting the crystal unit on a mounting board and electrically connecting the crystal unit to an external circuit. Joint member 4 for seam welding is disposed on the peripheral edge portion of the opening of the cavity in casing body 1. Joint member 4 is in the form of a metal ring of Kovar having its surface plated with gold (Au). Alternatively, joint member 4 is in the form of a thick metal layer comprising a metalized layer of tungsten (W) or the like directly disposed on the surface of the peripheral edge portion of the opening of the cavity in casing body 1 and a gold layer plated on the metalized layer, for directly welding metal cover 3 to casing body 1 which is made of laminated ceramics.
Crystal blank 2 comprises an AT-cut quartz crystal blank, for example, in the shape of a substantially rectangular plate, and has excitation electrodes (not shown) on both principal surfaces thereof. Extension electrodes are drawn from each of the excitation electrodes toward respective opposite ends of one side portion of the crystal blank. A pair of crystal terminals is disposed on the bottom of the cavity in casing body 1. The extension electrodes are secured to the crystal terminals by an electrically conductive adhesive, holding crystal blank 2 horizontally in the cavity. The crystal terminals are electrically connected to the external terminals by conductive paths formed through casing body 1.
Metal cover 3 includes core (i.e., base) 3A made of Kovar (Kv) which is a tertiary alloy of iron (Fe), nickel and cobalt (Co). Nickel layers functioning as barrier metal layers 6 are formed on both surfaces of metal cover 3 by cladding, for example. Kovar is used as core 3A of metal cover 3 because its thermal expansion coefficient is substantially equal to that of the laminated ceramics of casing body 1. Silver brazing layer 7, which is metal brazing material, is disposed on one of the principal surfaces of metal cover 3. Specifically, silver brazing layer 7 is disposed on barrier metal layer 6. Such a metal cover is joined to casing body 1 by seam welding, hermetically sealing crystal blank 2 in the cavity. Upon the seam welding, Joule's heat is generated in metal cover 3, especially in the core (Kovar) to melt the silver brazing layer, and then metal cover 3 is joined to casing body 1.
Kovar is corrosive because it is an alloy composed chiefly of iron, but is prevented from corroding by the nickel layer as barrier metal layer 6. Even if Kovar core 3A is corroded, the corrosion is stopped by the nickel layer, thereby reliably keeping crystal blank 2 hermetically sealed in the cavity.
A metal cover having a Kovar core used to seal an electronic component, and also an nickel layer and a brazing layer having a relatively low melting point, disposed on the Kovar core, is disclosed in Japanese laid-open patent publication No. 2000-164767 (JP, P2000-164767A). A casing body having a nickel layer, and a metal cover comprising a Kovar core and a nickel layer and a silver brazing layer which are disposed on the Kovar core, for use with a piezoelectric oscillation device such as a crystal unit, are disclosed in Japanese laid-open patent publication No. 2000-236035 (JP, P2000-236035A).
If a nickel layer is used as a barrier metal layer, then since nickel has a large electric resistivity, it is necessary to establish high voltage and large current conditions for seam-welding the metal cover to the casing body. Since such high voltage and large current conditions are liable to cause spark discharges, a rotational roller used in the seam welding process may be damaged, and the rotational roller or the like may be degraded early. Another problem is that an allowable range of current values is reduced, making it difficult to establish sealing conditions. Furthermore, inasmuch as the unit price of nickel is high, using nickel as the barrier metal results in an increase in the cost of the crystal unit.
If a nickel layer as barrier metal layer 6 is not provided, current will pass through gold layer, which is formed on the metallic ring or tungsten layer and has electric resistivity smaller than that of metal cover 3 (Kovar), upon application of voltage between a pair of the rotational roller used in the seam welding process, and then the current will generate Joule's heat to melt the silver brazing layer. However, the nickel layer as the barrier metal layer increases resistivity and limits the current in the seam welding process.
For absorbing thermal strains produced during the seam welding process, Japanese laid-open patent publication No. 2003-158211 (JP, P2003-153211A) discloses using a copper layer having a thickness in the range from 10 μm to 50 μm, preferably from 20 μm to 40 μm, instead of the nickel layer in the above arrangements, for stress relaxation. In this case, since copper has a relatively large thermal expansion coefficient, it is difficult to control the heating temperature for the copper layer and establish sealing conditions. When a copper layer is arranged between Kovar layer which is a core of metal cover 3 and the silver brazing layer, the silver brazing layer is heated via the copper layer and melts. Since copper is soft and has a large thermal expansion coefficient, expansion-shrinkage difference between the copper layer and the core or the casing body is generated between before and after the seam welding and impresses stress to the casing body. The strict control of heating temperature is necessary to prevent the casing body from being applied with the stress, and the establishment of sealing conditions becomes difficult.
For example, if current in a seam welding process is too large, the copper layer will lost its shape by the over-generated heat to run over to the outer side surface of the casing body. As a result, stress is generated in casing body by the expansion and shrinkage accompanied with temperature changes from the normal temperature to the heated condition and vice versa. The stress generated in casing body 1 may cause generation of cracks in casing body 1 and change in oscillation frequency due to change in holding conditions of the crystal blank by the conductive adhesive.
On the other hand, if the current in the seam welding process is too small, the silver brazing material does not sufficiently melt and the reliable hermetical sealing of the crystal unit may be deteriorated.
In any cases that a cupper layer is formed on a Kovar core, the establishment of sealing conditions of the metal cover becomes difficult due to the relatively large thermal expansion coefficient of copper.